1. Field of the Invention
The present invention relates to staggercasting methods and apparatus.
2. Background of the Invention
Current digital television transmission standards in the United States, as proposed by the Advanced Television Systems Committee (ATSC) dated Sep. 16, 1995, incorporated by reference herein, use a single carrier modulation technique: eight level vestigial sideband modulation (8-VSB). Because it is a single carrier modulation technique, it is susceptible to signal degradation in the communications channel, such as fading caused by multipath and other signal attenuation. While some such fading may be compensated by channel equalization techniques, if the fade is long enough and severe enough, then the receiver will lose the signal and the demodulator system will lose synchronization. Reacquiring the signal, and resynchronizing the demodulator can take several seconds and is quite objectionable to a viewer.
To overcome this problem, a first ATSC proposal permits creation of a second communications channel by permitting use of a more robust channel coding (modulation) technique for a limited period of time, e.g. less than 10%. For example, a 2 or 4-VSB modulation technique may be used for selected packets. A second ATSC proposal permits a more robust source encoding technique, e.g. trellis encoding, while maintaining an 8-VSB modulation technique. Such a system permits improved performance with compatible receivers while maintaining backwards compatibility with existing receivers.
Another known technique for overcoming fading is staggercasting. PCT Application No. US02/22723 filed Jul. 17, 2002, by K. Ramaswamy, et al., and PCT Application No. US02/23032 filed Jul. 19, 2002 by J. A. Cooper, et al., incorporated by reference herein, disclose staggercasting communications systems. Staggercasting communications systems transmit a composite signal including two component content representative signals: one of which is delayed with respect to the other. Put another way, one of the component content representative signals is advanced with respect to the other. The composite signal is broadcast to one or more receivers through a communications channel. At a receiver, the advanced-in-time component content representative signal is delayed through a delay buffer so that it becomes resynchronized in time with the other component content representative signal. Under normal conditions, the undelayed received component content representative signal is used to reproduce the content. If, however, a signal fade occurs, then the previously received and advanced-in-time content representative signal in the delay buffer is used to reproduce the content until either the fade ends and the composite signal is available again, or the delay buffer empties. If the delay period, and the associated delay buffer, is large enough then most probable fades may be compensated for.
Prior staggercasting communications systems permit a switch between the undelayed received content representative signal and the advanced-in-time received content representative signal to occur whenever a fade is detected and back again whenever the fade is over. However, should one of the component content representative signals have different video characteristics than the other one of the component content representative signals, then switching from one to the other may result in an abrupt visible change in the characteristics of the displayed video image, which may be objectionable to a viewer.
Further, in a video communications system, as proposed by the ATSC standard, the content representative signal is a video signal which is source encoded before transmission. This source coding generates coded segments. It is not possible to source decode a partial segment. Instead, the entire segment must be received to be source decoded properly. If a switch from one coded video signal to another is permitted to take place at any time, then it is possible, and indeed probable, that a switch will take place in the middle of transmitting a source coded segment. Thus, it will be impossible to source decode either the partially received segment switched from or the partially received segment switched to. The video signal source decoded from the received coded signal will be disrupted in a manner which will be visible and objectionable to a viewer.
A staggercasting system which can perform switching from one received coded signal to another, due to e.g. a fading event, without causing an objectionable artifact in the displayed video image is desirable.